Medical retrieval devices for capturing stones in a body tract generally include a basket. Some medical retrieval baskets are also mechanical lithotriptors which optionally crush or fragment stones that are too large to be removed intact from the body tract after the stone has been captured within the basket. One type of known device for crushing or breaking stones in a body tract has a wire basket that is typically introduced into a body tract via a working channel of an endoscope or by means of a guidewire.
Baskets designed for mechanical retrieval are typically constructed from a plurality of wires that are arranged and shaped such that their natural form, when unrestrained, is to expand radially outward. The basket wires are gathered together at their distal ends and at their proximal ends to form a basket. An elongated traction member typically extends from the basket through a sheath to a handle.
A typical retrieval basket is introduced into a body tract via an endoscope or catheter and maneuvered around the stone until the stone enters the basket. If it is necessary to reduce the size of the stone, it is then fragmented typically by applying tension to the basket wires surrounding the stone until sufficient force is applied directly to the stones by the basket wires to cause the stone to break apart.
Mechanical limitations and wide variation in the size, location, shape, and composition of stones in the body present problems in retrieving stones in a body tract. Methods that are used to reduce the size of the stone include lithotripsy such as by acoustic shock waves delivered to the stone from within or outside the body, laser energy applied directly to the stone, or compressive force applied to the stone by means of a mechanical basket. If mechanical basket lithotripsy is attempted on a stone composed of very hard material, the basket wire tension required to fragment the stone may exceed the strength of the basket wires, the various connecting joints of the medical device, the elongated traction member attached to the basket base, or the sheath. If the path of the body tract is very tortuous, the bends in the body tract will cause intimate contact of the traction member with the interior surface of the sheath. The friction generated between the traction member and the sheath will cause the tension applied to the proximal end of the traction member to be greatly reduced when delivered to the basket wires. It is possible for the mechanical strength of the basket wires, the various connecting joints in the device, the traction member, or the sheath to be exceeded even when fragmenting stones of only moderately hard composition.
Failure of the medical retrieval device, including the retrieval basket or any of its components, following capture of a stone may require release of the stone before the basket can be withdrawn from the body tract. Failure of a medical device may occur, for example, near the proximal end of a traction member impairing the ability of the operator to manipulate the basket from the proximal end of the device to remove the basket and captured stone from the body tract. Alternatively, the traction member may fail at the distal end of the device. The broken end of the traction member may traumatize the lining of the body tract if an attempt is made to withdraw the failed device from the body tract. If one or more of the basket wires fail, it may be impossible to remove the basket via the same route by which the basket was introduced into the body tract without traumatizing the lining of the body tract with the fragmented ends of the broken basket wires.
With most known retrieval baskets, it is difficult to disengage the stone from the basket so that the basket can be removed from the body tract. Retrieval baskets typically lack sufficient strength to break the stone. Attempts to perform lithotripsy may result in failure of the device, including failure of the retrievable basket or any of its components. If the stone can not be released from the basket, more invasive surgical approaches are required to disengage the stone from the basket and to remove the basket and stone from the body tract.